science is not engraved in stone, it evolves!
1) i see roundness as a good criteria, though not by itself alone.
2) completely clearing it's orbit of rubble, i don't know... there are some planets (of the 8 remaining) in our system that don't apply to that. also, new, young solar systems will probably not apply either. are those planetary bodys to be called proto-plantes?
3) also, i think the "dominant body" restriction is not defined clearly enough. i can imagine solar systems where several huge planets have excentric orbits that at some point intersect with others... what name to use then? bloody fat comets?
why is this definition important? well... who knows? Computer games aren't important to me, does that mean that they should be prohibited?? probably yes